Methods for drying and cleaning objects using aerosols

ABSTRACT

Methods for drying and cleaning objects that may have been wetted or contaminated in a manufacturing process. The objects are submerged in a rinse liquid in an enclosed chamber, and aerosol particles from a selected liquid are introduced into the chamber above the rinse liquid surface, forming a thin film on this surface. As the rinse liquid is slowly drained, some aerosol particles settle onto the exposed surfaces of the objects, and displace and remove rinse liquid residues from the exposed surfaces, possibly by a &#34;chemical squeegeeing&#34; effect. Surface contarminants are also removed by this process, which may be carried out at or near room temperature. Chamber pressure is maintained at or near the external environment pressure as the rinse liquid is drained from the chamber.

This patent application is a continuation of U.S. Ser. No. 08/624,689,filed Mar. 25, 1996, now abandoned, which is a continuation in part ofU.S. Ser. No. 08/616,165, filed Mar. 14, 1996, now U.S. Pat. No.5,685,086, which is a continuation in part of U.S. Ser. No. 08/484,921,filed Jun. 7, 1995, now U.S. Pat. No. 5,653,045.

FIELD OF THE INVENTION

This invention relates to drying and cleaning of manufactured objects,including electronic components, using aerosols created by sonic orultrasonic means.

BACKGROUND OF THE INVENTION

Objects that are being manufactured using processes involvingapplication of liquids and other fluids often require that the parts bethoroughly dried before the manufacturing process can continue. Forexample, in fabrication of integrated circuits, doping, photomasking,etching and passivation processes often require application ofparticular liquids at one stage and removal of liquid residues beforethe next stage proceeds. Drying and removal of these liquid residuesmust be complete, but the drying process should, ideally, occur in arelatively short time interval and with expenditure of a minimum ofenergy and chemicals to implement the drying process.

Several workers have disclosed methods for drying parts, includingintegrated circuits, by use of heated or superheated gases. McConnell etal, in U.S. Pat. No. 4,577,650, No. 4,633,983, No. 4,738,272, No.4,778,532, No. 4,856,844, No. 4,899,767, No. 4,911,761, No. 4,917,123and No. 4,984,597, disclose methods of drying semiconductor wafers byflowing a heated vapor or fluid past the wafers to be dried in a vessel,as part of a wafer processing sequence. The preferred drying vapor issuperheated isopropanol, which forms a minimum boiling azeotrope withwater and is believed to displace water from the wafer surfaces, and thevapor flows into the vessel at one end and simultaneously flows out ofthe vessel at another end.

In U.S. Pat. No. 5,383,484, Thomas et al disclose use of a plurality ofmegasonic beam transducers, located at staggered positions, for cleaningwafers. Each transducer emits a vibratory megasonic beam with anunspecified (very high) frequency in a fixed direction, and thetransducer locations are chosen so that the collection of beamsirradiate, and thereby clean, all wafer surfaces in a chamber, no matterhow the wafers are arranged.

Use of ultrasonic transducers in a chemical cleaning bath tocooperatively remove contaminants and unwanted material layers fromsemiconductor wafers, medical instruments and other objects of interestis disclosed by Erickson et al in U.S. Pat. No. 5,178,173, by Watanabeet al in U.S. Pat. No. 5,203,798, by Tamaki et al in U.S. Pat. No.5,227,001, by Evans et al in U.S. Pat. No. 5,248,456, by Smith et al inU.S. Pat. No. 5,337,446, by Koretsky et al in U.S. Pat. No. 5,368,054,by Steinhauser et al in U.S. Pat. No. 5,380,369, by Shibano in U.S. Pat.No. 5,447,171, by Awad in U.S. Pat. No. 5,464,477, by Kato in U.S. Pat.No. 5,467,791, by Thjietje in U.S. Pat. No. 5,468,302 and by Campbell inU.S. Pat. No. 5,472,005.

Use of ultrasonic transducers to coat, spray, deposit or otherwise applya desired material to an object surface is disclosed by Bachmann in U.S.Pat. No. 5,387,444, by Erickson et al in U.S. Pat. No. 5,409,163, and byVersteeg et al in U.S. Pat. No. 5,451,260. An ultrasonic fogging deviceis disclosed by Munk in U.S. Pat. No. 5,454,518.

These approaches heated or superheated gases or direct beam irradiationto dry an object surface; or they use cooperative action by anultrasonic beam and an active chemical bath to remove contaminants from,or to apply a desired material to, an object surface. These approachesare complex, usually require operation at high temperatures, oftenrequire processing times of several minutes, and often require use ofspecially resistant chamber walls for the processing chamber.

What is needed is a method and associated apparatus for drying andcleaning objects in a manufacturing process that works well at roomtemperature and is simple, that is demonstrably complete, with nosignificant residues, that can be accomplished in times as short as oneminute, that can be performed in a chamber with chamber walls made ofalmost any material, and that requires use of only a very small amountof a drying agent, with minimal expenditure of energy, particularlythermal energy. Preferably, the process should be performable over awide range of temperatures, and should be easily scalable to any sizesurface.

SUMMARY OF THE INVENTION

The needs are met by the invention, which provides a method andassociated apparatus for drying objects by use of recyclable water and asmall amount of a low surface tension liquid plus (optionally) briefapplication of a recyclable cleaning agent. In one embodiment, theobjects to be dried are submerged in a rinse liquid, such as water, in achamber. The rinse liquid surface is covered with a very thin film of alow surface tension selected liquid, such as isopropyl alcohol ("IPA"),formed from an aerosol created by sonic or ultrasonic vibrations of asmall stream of the selected liquid. Other suitable liquids includeethyl alcohol, methyl alcohol, tetrahydrofuran, acetone,perfluorohexane, hexane and ether. The thin film is continuallyreplenished as needed, and the rinse liquid covering the objects to bedried is slowly drained. As the rinse liquid and thin film drain, theselected liquid briefly contacts the surfaces of the objects and removeswater residues by a "chemical squeegeeing" process that is discussedlater. Optionally, the objects can be subjected to an additional chamberpurge or drying process, using a heated or ambient temperature cleaningfluid, such as dry N₂ CO or CO₂ gas, after the chamber has been drained.Optionally, chamber pressure is maintained near or above the externalenvironment pressure as the rinse liquid is drained from the chamber.

Process parameters that can be varied to control the process includevibration frequency for creation of aerosol particles from the selectedliquid, a representative aerosol particle diameter, delivery rate forthe selected liquid, pressure and temperature at which the selectedliquid is delivered for creation of the aerosol particles, temperatureof the drying fluid used (if any), and choice of the selected liquid andof the drying fluid used (if any).

The invention requires as little as 1-milliliters (ml) of the selectedliquid to dry objects in a chamber with volume of 10-20 liters, orsmaller or larger, if desired. This approach provides several benefits.First, the process is carried out at or near room temperature, withlittle energy expenditure, and does not require use of heated orsuperheated liquids or gases for drying. Second, the process uses a verysmall amount of the selected liquid in a large volume of rinse liquid(10-20 liters) so that the mixture of rinse liquid and selected liquidcan normally be disposed of without the special handling proceduresrequired for hazardous materials. Third, a wide variety of inexpensiveselected liquids can be used. Fourth, use of a covering film of selectedliquid minimizes vapor from the rinse liquid remaining in the chamberafter drainage. Fifth, the process is easily scaled up or down, with nosubstantial changes in the apparatus. Sixth, the process removes largediameter contaminants that are not chemically bound to an objectsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates suitable apparatus, in one embodiment, for practisingthe invention, with the objects submerged in a rinse liquid in achamber.

FIGS. 2A and 2B are schematic views of aerosol creating vibratingnozzles suitable for use with the invention.

FIG. 3 illustrates the apparatus of FIG. 1 with the rinse liquid partlydrained from the chamber.

FIG. 4 is a flow chart of one embodiment of the method.

DESCRIPTION OF BEST MODES OF THE INVENTION

FIG. 1 illustrates one embodiment of apparatus 10 that is useful forpractising the invention. An enclosed chamber 11 is defined by a housing12 and is provided with a rack (optional) for holding the objects 13A,13B, 13C, etc. to be dried. The objects 13A, 13B, 13C are placed into,and removed from, the chamber 11 through a slidable, hinged or otheroperable entryway 15 that is part of the housing 12. When the entryway15 is closed or engaged, the chamber is enclosed, preferably in angas-tight manner, and any remaining gas in the chamber can optionally beremoved. A first port 21 and associated first valve 23 are attached tothe housing 12 and are connected to a source 25 of water or othersuitable rinse liquid 27 in which the objects 13A, 13B, 13C areinitially submerged. A second port 31 and associated second valve 33 areattached to the housing 12 and are connected to a selected liquid source35, such as a pressurized tank maintained at a pressure of 5-50 psi, ofa selected drying liquid or fluid 37 ("selected liquid") that willprimarily dry the objects 13A, 13B, 13C.

A third port 41 and associated third valve 43, which may coincide withthe first port 21 and first valve 23, are attached to the housing 12 andare connected to a first liquid or fluid tank or other suitable firstdrain acceptor 45 that receives and drains the rinse liquid 27 andabsorbed selected liquid 37 from the chamber 11. A fourth port 51 andassociated fourth valve 53, which may coincide with the second port 31and second valve 33, are attached to the housing 12 and are connected toa second liquid or fluid tank or other suitable second drain means 55that receives and drains the selected liquid 37, and aerosol droplets 39from the selected liquid, from the chamber 11.

Initially, the objects 13A, 13B, 13C are placed in the chamber 11 in arack or cassette (not shown), the entryway 15 is closed or engaged, thechamber is evacuated, and rinse liquid 27 is admitted to the chamberthrough the first port 21 and first valve 23 so that the objects arefully submerged in the rinse liquid. The first valve 23 is then closed.Alternatively, the objects 13A, 13B, 13C may be partly submerged in therinse liquid 27 so that a portion of the surfaces of these objects areexposed above the exposed surface of the rinse liquid.

A small stream of the selected liquid 37 then passes through the secondport 31 and second valve 33 and is received by a piezoelectricallydriven head 61 and vibrating sonic or ultrasonic nozzle 63 that vibratesat a selected frequency f lying in the range 10 kHz≦f≦1000 kHz, and morepreferably in the narrower range 20 kHz≦f≦100 kHz. The driven head 61 isconnected to and driven by a frequency generator 64 that is preferablylocated outside the chamber 11 and that permits selection of a vibrationfrequency f in the indicated range. When the selected liquid 37 ispresent in the vibrating nozzle 63 and the nozzle is vibrating, theselected liquid is converted into a plurality of aerosol droplets 39that move into the chamber 11 and occupy most or all of an upper portion11U of the chamber that is not already filled by the rinse liquid 27 andsubmerged objects 13A, 13B, 13C.

FIG. 2A illustrates a suitable drive head 61A and vibrating nozzle 63Athat can be used with the apparatus shown in FIG. 1. The vibratingnozzle 63A preferably has a hollow column 65A formed therein withdiameter d(col)≈200 μm, through which the selected liquid 37(cross-hatched) flows. The vibrating nozzle then "shakes off" smalldroplets 39 of selected liquid 37, which form aerosol droplets in aroughly cylindrical pattern and move into the portion of the chamber 11above the rinse liquid.

FIG. 2B illustrates another suitable drive head 61B and vibrating nozzle63B, including a thin hollow column 65B therein through which theselected liquid 37 flows. A housing 67B surrounds the nozzle 63B anddirects a ring of hot or cold inert gas 69B toward the aerosol droplets39, which move into the chamber in a conical or other desired patternfor enhanced distribution of the aerosol droplets throughout thechamber. Many other drive head/vibrating nozzle combinations can also beused here.

I have found that use of a higher frequency f will tend to produceaerosol droplets 39 with a smaller mean diameter d(mean). For avibration frequency f in the range 20 kHz≦f≦100 kHz, I estimate that themean aerosol droplet diameter lies in the range 10 μm≦d(mean)≦50 μm. Themean droplet diameter can be varied by varying the diameter(s) d(mem) ofthe membrane apertures 66 and by varying the frequency f of vibration ofthe vibrating nozzle 63A or 63B.

The selected liquid 37 should be non-reactive with the objects 13A, 13B,13C and with the walls of the chamber 11 and should have a substantiallylower surface tension than the surface tension of the rinse liquid.Suitable selected liquids include isopropyl alcohol, ethyl alcohol,methyl alcohol, tetrahydrofuran, acetone, perfluorohexane, hexane andether, as well as many other low surface tension liquids and fluids. Useof any of these substances as a selected liquid does not requireprovision of chamber walls made of specially-resistant materials.

The selected liquid 37 may be held in the selected liquid source 35 at apressure of 5-50 psi above atmospheric pressure to facilitate deliveryand to suppress the slight volatilization of the selected liquid thatmight otherwise naturally occur. The preferred rinse liquid, de-ionizedwater, has a surface tension σ=73 dynes/cm at T≈20° C., and organicmolecules such as methyl alcohol, ethyl alcohol, isopropyl alcohol,n-hexane and ether have surface tensions σ in the range 17 dynes/cm≦σ≦23dynes/cm at T=20° C. so that σ(selected liquid)<<σ(rinse liquid) at roomtemperature.

Use of the selected liquid 37 at or near room temperature is preferredhere. Use of the selected liquid 37 at a substantially elevatedtemperature can reduce the surface tension of the rinse liquid 27,relative to the surface tension of the selected liquid 37, and thusinterfere with the chemical squeegee effect relied upon for thisprocess.

An aerosol particle is a cluster or collection of molecules of theselected liquid 37 that has not undergone a phase transformation into avapor form. Thus, the energy E(aerosol) (1.6 Watts for a typical sonichead, or less than 100 Joules/gm at a flow rate of 2 ml/min) required toconvert one gram of the selected liquid 37 into aerosol droplets 39,provided by the vibrating nozzle, is much less than the energy ofvaporization E(vapor) required to heat and convert one gram of theselected liquid 37 into its vapor form. We estimate that the ratioE(aerosol)/E(vapor) is less than 2 percent. Production of the aerosolparticles can be carried out at or near room temperature, and use of avery high temperature, such as T=60-200° C., is neither needed noradvisable for this process. Further, only a small amount of the selectedliquid 37, as low as 1-5 ml, is required for drying several objects 13A,13B, 13C in the chamber 11.

The aerosol droplets 39 move into the chamber 11, and many of thesedroplets settle onto an exposed surface 29 (preferably calm) of therinse liquid 27 as a thin film 30 having a varying thickness h(aerosol).An estimated time required to form this thin film 30 is 40-60 sec. Aportion of the aerosol droplets 39 that join the film 30 will diffuseinto the rinse liquid 27 so that, if this film is not replenished withadditional aerosol droplets, the film 30 will quickly and substantiallydisappear. Preferably, the volume flow rate r(sel) of the selectedliquid 37 to the vibrating nozzle 63 is adjusted so that the rate atwhich aerosol droplets 39 join the film 30 is sufficient to maintain orincrease a selected thickness h(aerosol) for the film. A preferred rangefor the film thickness h(aerosol) is 0.5 mm≦h(aerosol)≦5 mm, but thisthickness may be made larger by increasing the volume flow rate r(sel).For a chamber 11 having an exposed (upper) surface for the rinse liquid27 with an area of about 900 cm², a volume flow rate r(sel)=r2=1-5 mlper minute of the selected liquid 37 suffices. Usually, a volume flowrate r2=1-2 ml/min is high enough. The time required to drain thechamber at a drain rate of 5 mm/sec is about 20-40 sec for asemiconductor wafer 10-20 cm in diameter. Thus, very little of theselected liquid 37 is absorbed or diffuses into the rinse liquid 27 inthe course of the time interval (60-100 sec) required for establishmentof the film and draining of the chamber.

Because so little of the selected liquid 37 is used in the process, theselected liquid source 35 may have a relatively small volume, as littleas 20-25 ml, and the selected liquid source 35 may be located at aconsiderable distance, such as 1-4 meters, from the chamber 11. Thisenhances the safety of the process, where a selected liquid is used thathas a low flash point or that can initiate an explosion.

A very small amount of the selected liquid 37 will vaporize naturally atthe process temperature, preferably room temperature, based on theequilibrium vapor pressure coefficient of the selected liquid at thattemperature. This vaporized portion should be relatively small in theenclosed chamber 11 at room temperature, and the vapor portion of theselected liquid 37 will quickly come to equilibrium with the liquid filmand aerosol portions of the selected liquid 37. Use of a processtemperature much higher than room temperature would produce a selectedliquid 37 with a moderately higher equilibrium vapor pressurecoefficient and a comcomitantly higher amount of vapor from the selectedliquid. This natural vaporization of a small part of the selected liquid37 is not regarded as a useful part of the drying process.

After a film 30 of the aerosol droplets is established on the surface 29of the rinse liquid 27, which may require 40-60 sec, the rinse liquid 27is slowly drained from the chamber 11 through the third port 41 andthird valve 43 into the drain tank 45. Draining of the rinse liquid 27will require an estimated 20-40 sec for a chamber holding 10-20 litersof the rinse liquid 27. A preferred range for the drain rate r(drain) is3-10 mm/sec decrease in the height of the rinse liquid 27 in the chamber11, and r(drain)=5 mm/sec is a suitable drain rate for this process.Draining occurs slowly in order to preserve the thin film 30 of theselected liquid 37 at the otherwise-exposed surface 29 of the rinseliquid. As draining of the rinse liquid 27 proceeds, aerosol droplets 39continue to be produced by flow of a small stream of the selected liquid37 through the vibrating nozzle 63. The volume flow rate r(sel) of theselected liquid 37 may be adjusted toward higher or lowers values asdraining of the rinse liquid 27 (and absorbed aerosol particles 39)proceeds.

As the rinse liquid 27 drains from the chamber 11, the surfaces 14A,14B, 14C of the objects 13A, 13B, 13C are increasingly exposed above theexposed rinse liquid surface 29 and overlying film 30, and aerosoldroplets 39 in the upper part of the chamber 11U settle onto theseexposed surfaces 14A, 14B, 14C, as shown in FIG. 3. Also, a portion ofthe film 30 of the selected liquid 37 may settle on the exposed portionsof the object surfaces 14A, 14B, 14C, rather than moving with the rinseliquid 27 toward the third port 41. The selected liquid 37 is chosen tohave a much smaller surface tension σ(sel) than the surface tensionσ(rinse) of the rinse liquid 27. If the rinse liquid 27 is water, theassociated surface tension is σ(rinse)=73 dynes/cm at room temperature.In this instance, the selected liquid 37 may be isopropyl alcohol("IPA") or ethyl alcohol or methyl alcohol, with the respective surfacetensions of σ=21.7 dynes/cm, 22.6 dynes/cm, and 22.8 dynes/cm at roomtemperature. The selected liquid 37 is also chosen for its ability todisplace rinse liquid at whatever process temperature is used. Roomtemperature (T=20° C.), and even lower temperatures, can be used here.The process also works satisfactorily at somewhat higher temperatures.

As exposed portions of the object surfaces 14A, 14B, 14C receive theaerosol droplets 39 of the selected liquid 37, new films 16A, 16B, 16Cof the aerosol droplets 39 or selected liquid 37 form on these exposedportions. As draining of the rinse liquid 27 from the chamber 11proceeds, and after draining is completed, the selected liquid 37 in thefilms 16A, 16B, 16C displaces most or all of the rinse liquid 27 thatremains on the exposed portions of the object surfaces 14A, 14B, 14C, inlarge part because the surface tension σ(sel) of the selected liquid 37is much smaller than the surface tension σ(rinse) of the rinse liquid27. The rinse liquid 27 that is displaced by the selected liquid runsdown the exposed surfaces 14A, 14B, 14C of the objects 13A, 13B, 13C andis drained away with the bulk of the rinse liquid in the chamber. Theselected liquid 37 that forms a film on the surfaces 14A, 14B, 14C ofthe objects 13A, 13B, 13C also runs down these surfaces and is drainedaway with the bulk of the rinse liquid 27. The films 16A, 16B, 16C ofselected liquid 37 thus act as "chemical squeegees" in removing rinseliquid 27 and selected liquid 37 from the exposed surfaces 14A, 14B, 14Cof the objects 13A, 13B, 13C.

This chemical squeegeeing of the objects' exposed surfaces 14A, 14B, 14Chas another benefit. The process not only dries the objects' surfacesbut also removes most of the larger contaminant particles from thesesurfaces, if these contaminant particles are not chemically bound to thehost surfaces. I have examined some bare silicon surfaces before thechemical squeegeeing process is applied and have found a substantialnumber of contaminant particles with diameter at least 0.3 μm on thesesurfaces, as indicated in column (2) of Table 1. I have then applied thechemical squeegeeing process, have re-examined the same surfaces aftercompletion of the chemical squeegeeing process, and have found thenumber of contaminant particles is reduced after completion of thechemical squeegeeing process, as shown in column (3) of Table 1. Theseresults indicate that chemical squeegeeing alone removes 12-100 percentof the contaminant particles with diameters greater than 0.3 μm,depending on size.

                  TABLE 1                                                         ______________________________________                                        Chemical Squeegee Removal of Large Contaminant Particles                                   Particles before                                                                          Particles after                                      Particle Size                                                                              Chem. Squeegee                                                                            Chem Squeegee                                        ______________________________________                                        0.329-0.517 μm                                                                          8           7                                                    0.518-0.810  7           2                                                    0.811-1.270  7           2                                                    1.271-1.990  3           1                                                    1.991-3.130  6           1                                                    3.131-4.910  6           0                                                    ______________________________________                                    

At about the time the rinse liquid 27 becomes fully drained from thechamber 11 and the surfaces 14A, 14B, 14C of the objects 13A, 13B, 13Care fully exposed, the second port 31 and second valve 33 are closed,the vibrating nozzle 63 is shut down, and the fourth port 51 and fourthvalve 53 are opened. The remaining selected liquid 37, aerosol droplets39, rinse liquid 27, and any vapor from the rinse liquid and selectedliquid are then removed from the chamber 11 through the fourth port 51.This portion of the process may require another 10-20 sec. but may becontinued for a longer time interval, if desired, to completely removethe remaining selected liquid 37 and any remaining rinse liquid 27 fromthe films 16A, 16B, 16C and from the chamber 11. Drying of the objects13A, 13B, 13C is now substantially complete.

Optionally, hot or room temperature dry nitrogen N2, carbon monoxide CO,carbon dioxide CO₂ or other inert gas may be admitted into the chamber11 through a fifth port 71 and associated fifth valve 73 to purge thechamber 11 and/or clean any remaining substances from the exposedsurfaces 14A, 14B, 14C of the objects 13A, 13B, 13C. The hot purge gasis received by the chamber 11 from a purge gas tank 75 and is removedthrough a sixth port 81 and associated sixth valve 83 that may coincidewith the fifth port 71 and fifth valve 73, respectively. The hot purgegas is received from the chamber 11 in a spent purge gas tank 85 forrecycling, processing or disposal. This portion of the process, ifincluded, may require another 30-60 sec.

FIG. 4 is a flow chart indicating the process steps to be taken in oneembodiment of the invention. In step 91, the objects 13A, 13B, 13C(FIGS. 1 and 3) to be dried and/or cleaned are placed into the chamber,and the chamber is closed. In step 93, rinse liquid 27 is admitted intothe chamber to partially or (preferably) fully submerge the objects. Instep 95, aerosol droplets of the selected liquid 37 are formed withinthe chamber, and a film of the selected liquid is formed and maintainedon the exposed surface of the rinse liquid. In step 97, the rinse liquid27 and any absorbed selected liquid 37 are slowly drained from thechamber, to ultimately expose the surfaces of the objects to the aerosoldroplets and to allow films of the selected liquid to form on theobjects surfaces; optionally, the chamber pressure is maintained near orabove the external environment pressure. In step 99, the films ofselected liquid on the objects' surfaces perform chemically squeegeeingto remove any remaining rinse liquid 27 and remaining selected liquid 37and contaminants from the objects' surfaces. In step 101 (optional), anyremaining selected liquid 37 and rinse liquid 27 are removed from thechamber. In step 103 (optional), a purge gas is passed through thechamber to remove any remaining gas and/or liquid particles from thechamber. The objects, now dried and/or cleaned, can be removed from thechamber or may be further processed in the chamber.

No matter what drain rate r1 is selected, removal of the rinse liquid 27from the chamber 11 will produce a vacuum within the chamber that is notfully relieved by receipt within the chamber of the small amount ofselected liquid from the drive head 61 and vibrating nozzle 63. If thechamber 11 is sufficiently air-tight, little or no gas from the externalenvironment will enter the chamber in response to creation of thisvacuum. However, many chambers are not sufficiently air-tight; and insuch instances an appreciable amount of gas from the externalenvironment, possibly bringing with this gas one or more contaminantparticles that may settle on the exposed surfaces 14A, 14B, 14C of theselected objects 13A, 13B, 13C. This has been observed in some, but notall, of the tests of the procedure and apparatus disclosed here.

With reference to FIG. 3, a reservoir 121 of a substantially inertdisplacement gas 122, such as N₂, CO or CO₂, is optionally provided andis in fluid communication with the chamber 11. The inert gas 122 in thereservoir 121 passes through a port 123 and an associated valve andpressure control device 125 to enter the chamber 11. The valve andpressure control device 125 senses the increasing (negative) pressurethat is created within the chamber 11 as the rinse liquid 27 is drainedfrom the chamber using the port and valve 41 and 43. In response to thisincreasing (negative) pressure, the valve and pressure control device125 allows sufficient inert gas 122 from the inert gas reservoir 121 toenter the chamber so that the chamber pressure is maintained at apressure p≈p(external), where p(external) is approximately equal to thelocal pressure external to the chamber, or at a higher pressure. Achamber pressure p that is somewhat higher than the local externalpressure p(external) is preferred here so that some of the inert gas 122will tend to move out of the chamber 11 into the external environmentand will discourage in-flow of gases from the external environment, ifthe chamber is 2 0 not sufficiently air-tight. Optionally, the pressurep maintained within the chamber 11 may be somewhat less thanp(external), perhaps as low as 0.8p(external), and still discourageentry of gas from the external environment into the chamber. After therinse liquid 27 is fully drained from the chamber 11 and the surfaces14A, 14B and 14C of the selected objects 13A, 13B, 13C are fully driedand/or cleaned, the inert gas 122 may be removed from the chamber to aninert gas reservoir 127 before the next step is taken in processing theselected objects.

Alternatively, if the drain rate r1 for the rinse liquid 27 from thechamber 11 is controlled sufficiently well, the valve and pressurecontrol device 125 need not sense the internal pressure of the chamber11. In this approach, the valve and pressure control device 125 admitsinsert gas 122 at a programmed volume flow rate r3 from the inert gasreservoir 121, where the rate r3 is sufficient to maintain the internalpressure p≈p(external) or higher within the chamber 11 as the rinseliquid 27 drains from the chamber.

The temperature T of the inert gas 122 is preferably at or near thetemperature of the rinse liquid, which is usually room temperature orsomewhat colder or somewhat warmer. The purge gas reservoir 75 may alsoserve as the inert gas reservoir 121, with inclusion of the valve andpressure control device 125.

We claim:
 1. A method for removing at least one contaminant particlehaving diameter of at least 0.3 μm from at least one exposed surface ofan object, the method comprising the steps of:placing a selected objectto be cleaned in an enclosed chamber; admitting a sufficient amount of arinse liquid having a selected surface tension into the enclosed chamberso that the selected object is partly or fully submerged in the rinseliquid; admitting into the enclosed chamber, at a volume flow rate r2lying in a first selected volume flow range, a selected liquid that hasa surface tension that is substantially lower than the surface tensionof the rinse liquid; forming aerosol droplets of the selected liquidwithin the enclosed chamber; allowing a portion of the aerosol dropletsto form a film of the selected liquid on an exposed surface of the rinseliquid; draining the rinse liquid from the enclosed chamber at a volumeflow rate r1 lying in a second selected volume flow range, and allowingthe film of the selected liquid to form on exposed surfaces of theselected object; and allowing the film of selected liquid to displacethe rinse liquid on the exposed surfaces of the selected object and tothereby remove said at least one contaminant particle having a diameterof at least 0.3 μm from said at least one exposed surface of theselected object.
 2. The method of claim 1, further comprising the stepof choosing said selected liquid to be chemically substantiallyunreactive with said selected object.
 3. The method of claim 1, furthercomprising the step of selecting said rate r1 so that the depth of saidrinse liquid in said enclosed chamber decreases at a rate of between 3mm/sec and 10 mm/sec.
 4. The method of claim 1, further comprising thestep of selecting said rate r2 to lie in said first selected volume flowrange 1 ml/min≦r2≦5 ml/min.
 5. The method of claim 1, further comprisingthe step of forming substantially all of said aerosol particles withinsaid enclosed chamber without a change in a vapor phase of said selectedliquid.
 6. The method of claim 1, further comprising the step of formingsaid aerosol particles within said encloned chamber with an energyexpenditure of 1.6 Watts.
 7. The method of claim 1, further comprisingthe steps of:providing a measure of a pressure of an environment that isexternal to said enclosed chamber; admitting a selected displacement gasinto said enclosed chamber at the time said rinse liquid is beingdrained from said enclosed chamber; and controlling a rate of admissionof the selected displacement gas into said enclosed chamber so thattotal gas pressure within said enclosed chamber is at or above theexternal environment pressure while said rinse liquid is being drainedfrom said enclosed chamber.
 8. The method of claim 1, further comprisingthe step of performing, at a temperature that is approximately roomtemperature, at least one of said steps of forming said aerosolparticles, allowing said portion of said aerosol particles to form saidfilm of said selected liquid, draining said rinse liquid from saidenclosed chamber, allowing said film of said selected liquid to form onsaid exposed surfaces of said selected object, and allowing said film ofsaid selected liquid to displace said rinse liquid on said exposedsurfaces of said selected object.
 9. The method of claim 2, furthercomprising the step of choosing said selected liquid from a group ofsubstantially unreactive liquids consisting of isopropyl alcohol, ethylalcohol, methyl alcohol, tetrahydrofuran, acetone, perfluorohexane,hexane and ether.
 10. The method of claim 7, wherein said step offorming said aerosol droplets comprises the step of passing saidselected liquid through a vibrating nozzle that vibrates at a selectedfrequency f lying in the range 10 kHz ≦f≦1000 kHz.
 11. The method ofclaim 10, further comprising said step of selecting said frequency f tolie in a range 20 kHz≦f≦100 kHz.
 12. The method of claim 10, furthercomprising the step of selecting said frequency f so that at least oneof said aerosol droplets has an estimated diameter d(sel) that lies inthe range 10 μm≦d(sel)≦50 μm.